docs(cache): Clarify custom cache key behavior with single-file purge

[mbullock1986]

Cache Rules that set custom cache keys require API purge with headers/cookies rather than dashboard single-file purge. This clarification reduces customer escalations about why single-file purge does not invalidate cached resources.

Summary

Screenshots (optional)

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The change adheres to the documentation style guide.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

[cloudflare-docs-bot]

Review

💡 2 suggestions found in commit 599793d.

Fix in your agent

Fix the following review findings in PR #32107 (https://github.com/cloudflare/cloudflare-docs/pull/32107).

Before making changes, review each finding and present a brief summary table:
- For each finding, state whether you agree, disagree, or need clarification
- If you disagree (e.g. the fix requires disproportionate effort for minimal benefit,
  or the finding is factually incorrect), explain why
- If you need clarification before deciding, ask those questions
- Then share your plan for which issues to tackle and in what order

After triaging, fix all legitimate findings. For any you decide to skip,
post a comment on this PR with the finding ID and your reasoning.

---

## Style Guide Review

### Suggestions (2)

#### SG-12fd64d3f601 · Passive voice
- **File:** `src/content/docs/cache/how-to/cache-rules/index.mdx` line 35
- **Issue:** Line uses passive voice: "the resulting cache entry is indexed by that key".
- **Fix:** Rewrite in active voice, e.g. "that key indexes the cache entry rather than the request URL alone."

#### SG-c08c1dbeb9e7 · Passive voice
- **File:** `src/content/docs/cache/how-to/purge-cache/purge-by-single-file.mdx` line 32
- **Issue:** Added line uses passive voice: 'it is treated as an empty value in the cache key.'
- **Fix:** Rewrite in active voice, e.g., 'The cache treats any missing header or cookie as an empty value.'

Code Review

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_{No code review issues found.}

Conventions

_{No convention issues found.}

Style Guide Review

Suggestions (2)

File	Issue
cache/how-to/cache-rules/index.mdx line 35	Passive voice — Line uses passive voice: "the resulting cache entry is indexed by that key". Fix: Rewrite in active voice, e.g. "that key indexes the cache entry rather than the request URL alone."
cache/how-to/purge-cache/purge-by-single-file.mdx line 32	Passive voice — Added line uses passive voice: 'it is treated as an empty value in the cache key.' Fix: Rewrite in active voice, e.g., 'The cache treats any missing header or cookie as an empty value.'

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